This invention relates to a rotation sensor which detects the magnetic field which changes with the rotation of a rotary shaft, thereby to detect the amount of rotation of the rotary shaft.
The principle of detection with a rotation sensor of this type is as follows: A magnet, which is so magnetized that N and S poles are provided alternately as viewed in the circumferential direction, is secured to a rotary shaft so that it is rotated together with the latter. Under this condition, a magnetic sensor is used to detect the magnetic field of the magnet which changes with rotation of the rotary shaft, thereby to detect the station of rotation of the rotary shaft. That is, since the N and S poles are provided alternately as viewed in the circumferential direction, as the magnet rotates the direction of the magnetic field changes from circumferential direction to radial direction or vice versa at each of the boundaries between the N and S poles. This change is detected for the state of rotation of the rotary shaft.
In this case, the accuracy of detection for the state of rotation; that is, the resolving power can be increased by increasing the number of changes in magnetic field per revolution; that is, by increasing the number of magnetic poles formed in the magnet in the circumferential direction. In general, increasing the number of magnetic poles in a magnet will decrease the magnetic field strength per magnetic pole. This affects the sensitivity of detection of the magnetic sensor, and therefore the increasing of the number of magnetic poles makes it difficult to improve the accuracy of detection.
In order to eliminate this difficulty, the following method has been proposed in the art. As shown in FIG. 4, a magnet 1 is formed in such a manner that N and S poles appear alternately in a radial direction. More specifically, in the magnet 1, N and S poles are provided alternately both in the circumferential direction and in the radial direction, so that, in the detecting surface 2, the magnetic flux extends radially as well as circumferentially (as indicated by the arrows In FIG. 4). This increases the region where parallel magnetic fields are obtained, and, therefore, with the same magnetization, the magnetic field formed is higher. Hence, although the number of magnetic poles is increased, the change in the direction of magnetic field can be satisfactorily detected with the magnetic sensor; that is, the accuracy of detection of the magnetic sensor can be improved.
In the above-described conventional rotation sensor, the magnetization of the magnet 1 is carried out as follows: As shown in FIG. 5, a U-shaped magnetizing yoke 3 is employed to form N and S poles in the magnetizing surface 2 of the magnet alternately in a radial direction. However, since the surface 2 is small, it is considerably difficult to form N and S poles alternately therein with high accuracy, and accordingly it is difficult to accurately locate the neutral line M between N and S poles. Hence, when the magnet 1 is rotated, the change in magnetic field detected by the magnetic sensor 4 is not uniform. Thus, the output of the magnetic sensor is unstable, and accordingly the accuracy of detection is lowered as much.